The purpose of the notice of intended prosecution (NIP) is to inform the potential defendant that he may be prosecuted for the offence he has committed, whilst the incident is still fresh in his memory.
When you receive an NIP it does not automatically mean that you are going to face prosecution, it is a warning that you may face prosecution.
The NIP must be served within 14 days of the offence, otherwise the offence cannot proceed at court. If the details of the driver are not known then it is sent to the registered keeper. So long as the registered keeper is sent it within the time limit, the notice is valid. If the registered keeper has changed address/not informed DVLA etc., as long as the NIP was posted to arrive within 14 days, it is still valid. The registered keeper then has an obligation to identify the driver.
The driver, may then receive further paperwork in due course, but that is not to be confused with the document that is legally required to be sent within the 14 days.
NIPs can also be issued verbally to the driver at the time of the offence or alternatively you could receive a court citation through the post for the alleged offence within the 14 days.
Small mistakes on the notice do not render it ineffective unless it would mislead the potential defendant.
The posted NIP is deemed to be served until the contrary is shown. There is a presumption that it arrived, however it is possible for a potential defendant or other witness to satisfy the court (on the balance of probabilities) that neither he/she (nor the registered keeper where applicable) received the notice